Quantum Dot Nanoparticles Having Enhanced Stability and Luminescence Efficiency

ABSTRACT

Certain dithio-compounds have been found to be superior capping ligands for quantum dot (QD) nanoparticles. Example dithio-ligands include dithiocarbamate ligands. These strongly binding ligands are capable of coordinating to both positive and negative atoms on the surface of the nanoparticle. The ligands are bi-dentate and thus their approach to the QD surface is not as sterically hindered as is the approach of mono-dentate ligands. These ligands can therefore completely saturate the QD surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional applicationSer. No. 14/615,243 filed on Feb. 5, 2015, which claims the benefit ofU.S. Provisional Application No. 61/937,073 filed on Feb. 7, 2014.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to semiconductor nanoparticles(“quantum dots”). More particularly, it relates to semiconductornanoparticles having capping ligands on their outer surface.

2. Description of the Related Art Including Information Disclosed Under37 CFR 1.97 and 1.98.

There has been substantial interest in the preparation andcharacterization of compound semiconductor particles with dimensions onthe order of 2-100 nm, often referred to as quantum dots (QDs) and/ornanocrystals. This is mainly because of their size-tunable electronic,optical and chemical properties. For example, many QDs displayrelatively strong emission in the visible region of the electromagneticspectrum. Moreover, the wavelength of light absorbed and emitted is afunction of the size of the QD. Because of their unique opticalproperties, QDs are promising materials for commercial applications asdiverse as biological labeling, solar cells, catalysis, biologicalimaging, light-emitting diodes amongst many new and emergingapplications.

To date, the most studied and prepared of semiconductor materials havebeen the II-VI materials, namely, ZnS, ZnSe, CdS, CdSe, CdTe—mostnotably CdSe due to its tuneability over the visible region of thespectrum. As mentioned semiconductor nanoparticles are of academic andcommercial interest due to their properties, which are unique from theproperties of the corresponding crystalline bulk forms of the samesemiconductor materials. Two fundamental factors, both related to thesize of the individual nanoparticles, are responsible for their uniqueproperties. The first is the large surface-to-volume ratio. As aparticle becomes smaller, the ratio of the number of surface atoms tothose in the interior increases. This leads to the surface propertiesplaying an important role in the overall properties of small particles.The second factor is that, with semiconductor nanoparticles, there is achange in the electronic properties of the material with the size of theparticle. Specifically, the band gap gradually becomes wider as the sizeof the particle decreases. This change in band gap is because of quantumconfinement effects. This effect is a consequence of the confinement ofan “electron in a box,” giving rise to discrete energy levels similar tothose observed in atoms and molecules, rather than a continuous band asin the corresponding bulk semiconductor material. Thus, for asemiconductor nanoparticle, the “electron and hole” produced by theabsorption of a photon are closer together than in the correspondingmacrocrystalline material, resulting in non-negligible Coulombicinteraction between the electron and hole. This leads to a narrowbandwidth emission that is dependent upon the particle size andcomposition. Consequently, quantum dots have higher kinetic energy thanthe corresponding macrocrystalline material and the first excitonictransition (band gap) increases in energy with decreasing particlediameter. Thus, quantum dots with a smaller diameter absorb and emitlight of higher energy than do quantum dots with a larger diameter. Inother words, the color of light absorbed and emitted can be “tuned” as afunction of the particle diameter.

Single core nanoparticles, which consist of a single semiconductormaterial, tend to have relatively low quantum efficiencies due toelectron-hole recombination occurring at defects and dangling bondssituated on the nanoparticle surface that lead to non-radiativeelectron-hole recombinations. One method to eliminate defects anddangling bonds is to grow a shell of a second semiconductor materialhaving a wider band-gap on the surface of the core particle to produce a“core-shell particle”. The shell semiconductor material preferably has asmall lattice mismatch with the core material so that the interfacebetween the two materials is minimized. Core-shell particles separatecharge carriers confined in the core from surface states that wouldotherwise act as non-radiative recombination centers. A common exampleis ZnS grown on the surface of CdSe cores. Excessive strain can furtherresult in defects and non-radiative electron-hole recombinationresulting in low quantum efficiencies.

Several synthetic methods for the preparation of semiconductornanoparticles have been reported. Early routes applied conventionalcolloidal aqueous chemistry, while more recent methods involve thekinetically controlled precipitation of nanocrystallites, usingorganometallic compounds.

Since the optical properties of QDs are size-dependent, it is oftendesirable to produce populations of QDs with a high degree ofmonodispersity, i.e., with a high degree of uniformity in the size ofthe QDs in the population. Also, populations of QDs with a high quantumyield (QY, the ratio of photons emitted to photons absorbed) aredesirable. Methods have been reported to produce semiconductor QDs withhigh monodispersity and with quantum yields greater than 50%. Most ofthese methods are based on the original “nucleation and growth” methoddescribed by Murray, Norris and Bawendi, M. G. J. Am. Chem. Soc. 1993,115, 8706. Murray et al. originally used organometallic solutions ofmetal-alkyls (R2M) M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphinesulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP).These precursor solutions are injected into hot tri-n-octylphosphineoxide (TOPO) in the temperature range 120-400° C. This produces TOPOcoated/capped semiconductor nanoparticles of II-VI material. The size ofthe particles can be controlled by the temperature, concentration ofprecursor used, and length of time of the synthesis. This organometallicroute has advantages over other synthetic methods, including nearmonodispersity and high particle crystallinity.

Cadmium and other restricted heavy metals used in conventional QDs arehighly toxic and represent a major concern in commercial applications.The inherent toxicity of cadmium-containing QDs prevents their use inany applications involving animals or humans. For example recent studiessuggest that QDs made of a cadmium chalcogenide semiconductor materialcan be cytotoxic in a biological environment unless protected.Specifically, oxidation or chemical attack through a variety of pathwayscan lead to the formation of cadmium ions on the QD surface that can bereleased into the surrounding environment. Although surface coatingssuch as ZnS can significantly reduce the toxicity, it may not completelyeliminate it because QDs can be retained in cells or accumulated in thebody over a long period of time, during which their coatings may undergosome sort of degradation exposing the cadmium-rich core.

The toxicity also affects applications including optoelectronic andcommunication because heavy metal-based materials are widespread in manycommercial products including household appliances such as IT andtelecommunication equipment, lighting equipment, electrical andelectronic tools, toys, leisure and sports equipment. Legislation torestrict or ban certain heavy metals in commercial products has beenalready implemented in many regions of the world. For example, theEuropean Union directive 2002/95/EC, known as the “Restrictions on theuse of Hazardous Substances in electronic equipment” (or RoHS), bans thesale of new electrical and electronic equipment containing more thancertain levels of lead, cadmium, mercury, hexavalent chromium along withpolybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE)flame retardants. As a result of this mandate, manufacturers have had tofind alternative materials and develop new engineering processes for thecreation of common electronic equipment. In addition, on 1 Jun. 2007, aEuropean Community Regulation came into force concerning chemicals andtheir safe use (EC 1907/2006). The Regulation deals with theRegistration, Evaluation, Authorization and Restriction of Chemicalsubstances and is known as “REACH”. The REACH Regulation imposes greaterresponsibility on industry to manage the risks from chemicals and toprovide safety information on the substances. It is anticipated thatsimilar regulations will be extended worldwide. Thus, there issignificant economic incentive to develop alternatives to II-VI QDmaterials.

Due to their increased covalent nature, III-V and IV-VI highlycrystalline semiconductor nanoparticles are more difficult to prepareand much longer annealing times are usually required. However, there arenow reports of III-VI and IV-VI materials being prepared in a similarmanner to that used for the II-VI materials. Examples of such III-VI andIV-VI materials include GaN, GaP, GaAs, InP, InAs and for PbS and PbSe.

For all of the above methods, rapid particle nucleation followed by slowparticle growth is essential for a narrow particle size distribution.All these synthetic methods are based on the original organometallic“nucleation and growth” method by Murray et al., which involves therapid injection of the precursors into a hot solution of a Lewis basecoordinating solvent (capping agent). The addition of the coolersolution lowers the reaction temperature and assists particle growth butinhibits further nucleation. The temperature is then maintained for aperiod of time, with the size of the resulting particles depending onreaction time, temperature, and ratio of capping agent to precursorused. The resulting solution is cooled followed by the addition of anexcess of a polar solvent (methanol or ethanol or sometimes acetone) toproduce a precipitate of the particles that can be isolated byfiltration or centrifugation. Generally, larger particles precipitateeasier than smaller particles. Thus, precipitation provides a means ofseparating the quantum dots as a function of their size. Multipleprecipitation steps are required to achieve narrow particle sizedistributions.

Fundamentally, these prior art preparations rely on the principle ofparticle nucleation followed by growth. That principle relies onseparation of the nanoparticle nucleation step (at a high temperature)from the nanoparticle growth step (at a lower temperature) to obtainmonodispersity. Such separation of steps is achieved by rapid injectionof one or both precursors into a hot coordinating solvent (containingthe other precursor if not otherwise present), which initiates particlenucleation. The sudden addition of the cooler solution upon injectionsubsequently lowers the reaction temperature (the volume of solutionadded is about ⅓ of the total solution) and inhibits further nucleationmaintaining a narrow nanoparticle size distribution. This method maywork well for small-scale synthesis where one solution can be addedrapidly to another while keeping a homogenous temperature throughout thereaction. However, on a larger preparative scale, whereby large volumesof solution are required to be rapidly injected into one another,temperature differentials can occur within the reaction, which can leadto a large particle size distribution. Moreover, the need to performmultiple size-selective purification steps is not practical for theproduction of large quantities of QDs.

U.S. Pat. Nos. 7,588,828, 7,803,423, 7,985,446, and 8,062,703(collectively referred to herein as “the seeding patents”), the entirecontents of which are hereby incorporated by reference, describesynthetic methods for preparing monodisperse QD populations that do notrely on the hot injection methods and the size-selective purificationsteps described above. Briefly, the methods involve the use of amolecular cluster “seed” compound that serves as a template for thenucleation of the QD semiconductor material in solution. The clustercompound acts as a seed or nucleation point upon which nanoparticlegrowth can be initiated. In this way, a high temperature nucleation stepis not necessary to initiate nanoparticle growth because suitablenucleation sites are already provided in the system by the molecularclusters. By providing nucleation sites that are more uniform than thenucleation sites employed in the methods described above, the synthesisprovides a population of QDs that are essentially monodisperse. Asignificant advantage of the molecular seeding method is that it can beeasily scaled-up.

Regardless of how the QD nanoparticles are prepared, the bonding of theatoms about the outer inorganic surface atoms is incomplete. The surfaceis populated with highly reactive “dangling bonds”, which can lead toparticle agglomeration. These uncoordinated surface atoms may alsoprovide excitons with surface states that can give alternativerecombination pathways to radiative emission. Such pathways areundesirable and lead to lower luminescence. Additionally, uncoordinatedatoms can be more susceptible to oxidation.

The problems associated with uncoordinated dangling bonds can bepartially overcome by passivating (capping) the “bare” surface atomswith protecting organic groups. The capping or passivating of particlesnot only prevents particle agglomeration from occurring, it alsoprotects the particle from its surrounding chemical environment andprovides electronic stabilization (passivation) to the particles. Thecapping agent is typically a Lewis base compound or otherelectron-donating compound that binds to surface metal atoms of theoutermost inorganic layer of the particle (for example, bound to theoutermost shell of a core-shell QD particle). When the QD shellsynthesis is performed in an electron-donating solvent, such asTOP/TOPO, the capping agent may simply be solvent molecules adhered tothe surface of the QD. In the case that a non-electron-donating solventis used, electron-rich capping agent may be added to the shellsynthesis. For example, if a solvent such as THERMINOL is used for theshelling reaction, it may be necessary to add an electron-donatingcompound, such as myristic acid to the reaction mixture to provide acapping ligand.

While electron-donating capping ligands provide some stability andsurface passivation, many of these ligands only weakly adhere to thesurface of QD nanoparticles. Desorption of the capping ligands leavevacancies on the surface that can lead to agglomeration, precipitation,and that are detrimental to the quantum efficiency of the QDs. One wayof addressing the problem of weakly bound capping ligands has been touse capping ligands that contain functional groups that have a specificbinding affinity for atoms on the surface of the QDs. For example, thesulfur of thiol compounds has an affinity for many of the metal atoms,such as zinc, that are commonly components of QD shell semiconductormaterials, such as ZnS and ZnSe. Thus, thiols have been widely used ascapping ligands for QDs. But thiol capping ligands can also desorb,leaving problematic vacancies on the QD surface. One probable mechanismfor thiol ligand desorption is via the formation of disulfide bondsbetween neighboring thiols on the QD surface, followed by desorption ofthe disulfide. Another problem with thiol capping ligands is that, insome cases, steric hindrances may prevent complete surface coverage. Inother words, once a certain surface coverage is obtained, additionalthiol molecules are sterically prevented from reaching the surface ofthe QD to bond even though there are still a significant number ofdangling bonds at the surface left unfilled. There is thus a need formore affective capping ligands for QDs in order to maximize theperformance and stability of QD materials.

BRIEF SUMMARY OF THE INVENTION

The dithio-ligands disclosed herein address the problems describedabove. Example dithio-ligands include dithiocarbamate ligands. Thestrongly binding ligands described herein are capable of coordinating toboth positive and negative atoms on the nanoparticle's surface. Theligands are bi-dentate and thus their approach to the QD surface is notas sterically hindered as is the approach of mono-dentate ligands. Thedisclosed ligands can therefore completely saturate the QD surface.

According to one embodiment, a first ligand having a group that bindsthe electropositive atoms of the QD surface is added to the QD and thena second ligand having a group that binds the electronegative atoms isadded. For example, if the outermost shell comprises ZnS, one ligand maypreferentially bind to Zn atoms, which are electropositive on the shellsurface and a second ligand may preferentially bind to S atoms, whichare more electronegative. By binding a greater number of the availablebinding sites on the QD surface, the dithiocarbamate ligands disclosedherein enhance the stability and optical characteristics of QDnanoparticles.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a flow diagram of a process according to the invention forpreparing QDs with dithiocarbamate ligands.

DETAILED DESCRIPTION OF THE INVENTION

The dithiocarbamate ligands described herein can be used as cappingligands for generally any type of semiconductor QD nanoparticle.Particularly useful QD nanoparticles are prepared as described in U.S.Pat. Nos. 7,588,828, 7,803,423, 7,985,446, and 8,062,703 (referred toherein collectively as “the seeding patents”), the entire contents ofwhich are hereby incorporated by reference. FIG. 1 illustrates such amethod of preparing QD nanoparticles. Briefly, the method involvesreacting core material precursors in the presence of a molecular clustercompound, which acts as a seed for crystal growth (101). The seedingpatents describe a number of precursor materials and molecular clustercompounds. As an example, suitable precursor compounds for an InP-basedcore must provide a source of indium and a source of phosphorus. Forexample, the indium source may be indium myristate and the phosphorussource may be tris(trimethylsilyl)phosphine. It will be appreciated thatother indium and phosphorus sources may be used.

The core-forming reaction may be conducted in the presence of amolecular seeding compound. Suitable molecular seeding compounds aredescribed at length in the seeding patents referenced above. One exampleof a suitable molecular seeding compound is the zinc sulfide-basedmolecular seeding compound described in U.S. Pat. No. 8,062,703. Thecore precursor compounds and the molecular seeding compound are heatedin a solvent under conditions described in the seeding patents and U.S.Patent Publication No. 2010/0068522, the entire contents of which arehereby incorporated by reference. Generally, a non-electron donatingsolvent is used for the reaction. One example of an appropriate solventis a heat transfer fluid such as THERMINOL® (SOLUTIA INC., ST. LOUISMISSOURI 63141).

It may be desirable to monitor core growth 102 (e.g., via an opticalproperty of the QD core) during the synthesis of the core. For example,the absorbance spectrum may be monitored as the QD core grows and thereaction may be stopped when the core reaches the proper size to yieldthe desired absorbance and/or emission spectrum. Once the desirableoptical value is obtained and the reaction stopped, the cores can beisolated 103, for example, by filtration. It may be desirable to add anon-solvent to the reaction mixture to induce precipitation of thecores. Once the cores are isolated, they may be reacted with shellprecursors 104 to grow one or more semiconductor shells on the cores. Itmay be desirable to pretreat the cores, for example, by etching a smallamount of the material from the core, prior to reacting the core withthe shell precursors. An acid, such as hydrofluoric acid may be used toetch the core.

The shell precursors react to form a shell of semiconductor material onthe QD cores. If a non-coordinating solvent is used during the shellingreaction it may be necessary to add a coordinating ligand such asmyristic acid during the shelling reaction to stabilize the shellingreaction. If a coordinating solvent such as TOP/TOPO is used, thensolvent molecules themselves may act as ligands to stabilize theshelling reaction. In either case, the outermost semiconductor surfaceof the QDs is bound to capping ligands.

The next step 105 is to exchange at least a portion of the cappingligands with the disclosed dithio-ligands. The dithio ligands have thegeneral structure (1):

Examples of dithio-ligands include dithiocarbamate ligands. Examples ofsuitable first dithiocarbamate capping ligands are represented bystructures (2) and (3): (2) (3)

Other suitable dithio-ligands include alkyldithiocarbonate (i.e.,xanthate) ligands, having the general structure (4):

The R groups (that is, R and R′) may be any functional groups, and aregenerally hydrocarbons. Examples include alkyl or aryl groups. Oneexample is ethyl groups. According to some embodiments, the R groups maycontain functional groups with the ability to tailor the hydrophilicityof the QDs, for example to render the QDs more soluble in hydrophilicenvironments. An example of such an R group is a C12 hydrocarbon chainwith a carboxylic acid functionality on the C12 position. Once suitablycoordinated to the surface of the dots, the carboxylic acid can bedeprotonated with a base and the dots transferred to hydrophilic media.Alternatively, the R groups may be an amphiphilic group such as a PEG(polyethyleneglycol).

A first dithio-capping ligand may be exchanged to bind to theelectropositive atoms on the surface. For example, the firstdithio-capping ligand may bind to zinc on the surface of a ZnS or ZnSesurface. The first dithio-capping ligand compounds (1)-(4) may beprovided as a salt of sodium or potassium. The dithio-ligands may beprovided as a powder or in a solvent such as THERMINOL.

Another example of a suitable dithio-ligand is represented by structure(5):

In structure (5), R is as defined above and R′ is typically an alkylgroup. Compounds of structure (5) may exist in both monomeric anddimeric forms. When R′ is an ethyl group, the monomeric form is a majorspecies. Compounds according to structure (5) have great potential forcoordinating to surface sulfide and/or selenium ions, thereby satisfyingthe preferred coordination of the metal (Zn) atom.

Zinc dithiocarbamates, such as illustrated in structure (5) above, havebeen used as single source precursors for ZnS nanoparticles. Thosecompounds are viable candidates for single source precursors fordepositing ZnS shells upon InP core nanoparticles. However, zincdithiocarbamates require high temperatures to make them decompose. Suchhigh temperatures negatively impact the performance of the InP corenanoparticle. The decomposition temperature is significantly lower ifthe zinc dithiocarbamates are provided in the presence of amines.However, addition of amines to InP nanoparticles has a particularlydrastic quenching effect on radiative efficiency.

The Applicants have discovered that zinc dithio compounds, such as zinccomplexes of structures (2), (3), and (4), and structure (5) may be usedas single-source shelling precursors (i.e., single source precursorsproviding elements of a ZnS shell upon a core nanocrystal) if the zincdithiocarbamates are “pre-coordinated” to an amine. Another suitableshelling precursor is amine-coordinated zinc alkylthiophosphate, i.e.,amine-coordinated compounds of dithio-ligands having the followingstructure (6):

Pre-coordinating an amine with the zinc dithiocarbamate allows the amineand the zinc dithiocarbamate to be introduced into the solution ofnanoparticle cores during shelling at a 1:1 ratio of amine to zincdithiocarbamate. Thus, no free amine is available to participate inquenching reactions with the core surface. The decomposition temperatureof the zinc dithiocarbamate is suitably lowered by the amine, butwithout the adverse quenching effects observed when free amine ispresent. Examples of suitable amines include amines having longhydrocarbon constituents. A particularly suitable amine is oleylamine.

Examples Example 1

The quenching effect of free amine was demonstrated by shelling InPalloy-based cores in the presence and in the absence of oleylamine.InPZnS alloy-based cores were prepared essentially as described in U.S.Pat. No. 7,558,828. Two samples of InP alloy-based cores were eachsuspended in THERMINOL. Zinc acetate (0.862 g) was added to each mixtureand the mixtures were heated to 230° C. for 2 hours. Dodecanethiol (1.69mL) was added to both mixtures and oleylamine (0.2 mL) was added to oneof the mixtures. Both mixtures were allowed to react for a further 1½hours.

The quantum dots shelled in the absence of oleylamine exhibitedphotoluminescence at 644 nm having a full width at half maximum (FWHM)of 94 nm and a quantum yield (QY) of 75%. The quantum dots shelled inthe presence of oleylamine exhibited photoluminescence at 644 nm havinga FWHM of 90 nm and a quantum yield (QY) of 62%. The lower QY of thesample shelled in the presence of amine illustrates the quenching effectof the amine.

Example 2

InPZnS alloy-based cores were prepared as generally described in U.S.Pat. No. 7,588,828. The cores were suspended in THERMINOL (40 mL), towhich was also added zinc acetate (5.76 g), myristic acid (1.5 g), zincstearate (11.32 g). The mixture was heated at 180° C. and then zincdiethyldithio-carbamate (2.295 g) was added and left for 20 minutes.High turbidity was observed and the photoluminescence of the reactionmixture remained very low, suggesting low reactivity/solubility at thattemperature. The mixture was heated to 230° C. and left for 3 hrs andgradually the powder dissolved but the QY remains very low throughout.

Example 3

InPZnS alloy-based cores were shelled under the same conditionsdescribed in Example 2, except that oleylamine (5 mL) was added to themixture following the addition of diethyldithiocarbamate at 180° C. Themixture was heated for an additional 20 minutes at 180° C., yieldingquantum dots having a photoluminescence peak at 523 nm with a FWHM of 53nm and a QY of 73%. The difference between Examples 2 and 3 suggest thatamine is critical to shelling.

Example 4

InPZnS alloy-based cores prepared as generally described in U.S. Pat.No. 7,588,828 were shelled as described in Example 3, except that thezinc diethyldithiocarbamate was first complexed with oleylamine bypremixing the two together under nitrogen using a water bath set to 50°C. prior to adding to the shelling reaction. The shelling reactionyielded quantum dots having luminescence QY of 75%. Similar reactionsyielded quantum dots having luminescence QY as high as 85%.

Example 5

InPZnS alloy-based cores prepared as generally described in U.S. Pat.No. 7,588,828 were shelled as described in Example 4, except that zincethlyxanthate was used in the place of zinc diethyldithiocarbamate.InPZnS alloy-based cores (500 mg) myristic acid (1.5 g), zinc acetate(2.4 g), and zinc stearate (5.3 g) were stirred under vacuum inTHERMINOL at 100° C. for 1 hour and then heated to 215° C. beforecooling to 140° C. A premixed solution of zinc ethylxanthate (0.97 g)and oleylamine 1.05 (mL) in THERMINOL (5 mL) was added to the coresolution. The solution was stirred for 1 hr then at 180° C. for onehour. Octanol (2.4 mL) was added and the solution was stirred anadditional 30 minutes. The reaction yielded quantum dots havingluminescence QY of 85%.

Although particular embodiments of the present invention have been shownand described, they are not intended to limit what this patent covers.One skilled in the art will understand that various changes andmodifications may be made without departing from the scope of thepresent invention as literally and equivalently covered by the followingclaims.

What is claimed is:
 1. A method for preparing quantum dot (QD) nanoparticles comprising a plurality of first dithiocarbamate capping ligands comprising the steps of: reacting a core nanoparticle with shell precursors to grow at least one semiconductor shell on the core, to form a core-shell nanoparticle; binding capping ligands to an outermost semiconductor surface of core-shell nanoparticle; and exchanging at least a portion of the capping ligands with the dithiocarbamate capping ligands.
 2. The method recited in claim 1 further comprising monitoring an optical property related to the size of the cores and stopping the reaction when the cores reach a preselected size.
 3. The method recited in claim 2 wherein the optical property is an absorbance spectrum.
 4. The method recited in claim 2 wherein the optical property is an emission spectrum.
 5. The method recited in claim 1 further comprising etching the core prior to reacting the core with the shell precursors.
 6. The method recited in claim 5 wherein hydrofluoric acid is used to etch the cores.
 7. The method recited in claim 1 further comprising adding a coordinating ligand during the shelling reaction to stabilize the shelling reaction.
 8. The method recited in claim 7 wherein the coordinating ligand is myristic acid.
 9. A method for preparing core-shell nanoparticles comprising a plurality of first dithiocarbonate capping ligands comprising the steps of: reacting a core nanoparticle with shell precursors to grow at least one semiconductor shell on the core, to form a core-shell nanoparticle; binding capping ligands to an outermost semiconductor surface of core-shell nanoparticle; and exchanging at least a portion of the capping ligands with dithiocarbonate capping ligands.
 10. The method recited in claim 9 further comprising monitoring an optical property related to the size of the cores and stopping the reaction when the cores reach a preselected size.
 11. The method recited in claim 10 wherein the optical property is an absorbance spectrum.
 12. The method recited in claim 10 wherein the optical property is an emission spectrum.
 13. The method recited in claim 9 further comprising etching the core prior to reacting the core with the shell precursors.
 14. The method recited in claim 13 wherein hydrofluoric acid is used to etch the cores.
 15. The method recited in claim 9 further comprising adding a coordinating ligand during the shelling reaction to stabilize the shelling reaction.
 16. The method recited in claim 15 wherein the coordinating ligand is myristic acid. 